Abstract

The characteristics of tight oil reservoir are low porosity and ultra-low permeability, thus stimulated reservoir volume (SRV) should be conducted whether applying the mode of vertical wells or horizontal wells production. Tight oil reservoir is mostly developed by natural depletion or water flooding recently, but the problems are existed, including low recovery factor with natural depletion and the difficulty of water injection. To further improve the development effect of tight oil reservoir, CO2 flooding is proposed. Based on changing tight oil reservoir in Ordos Basin, an oil sample of typical block is selected. The PVT experiments are conducted. The compositional numerical model of five-spot pattern is established with a horizontal well in the middle and 4 vertical wells on the edge. Based on the model, several CO2 flooding scenarios of horizontal well with different completion measures are studied. Furthermore, parameters such as the formation pressure, production rate, shut-in gas-oil ratio and total gas injection volume are optimized. The results of this study show that the recovery factor of horizontal well with SRV is higher than those of horizontal well and conventional fractured horizontal well. The study gives new ideas of CO2 flooding with volume fractured horizontal well for the Ordos Basin tight oil reservoir. It can be helpful for rapid and effective development of tight oil reservoirs in Ordos Basin.

Keywords

CO2 flooding; Tight oil reservoir; Gas flooding

Introduction

In recent years, it is become a hot spot to develop unconventional reservoirs, such as tight oil, tight gas and shale gas [1,2]. Tight oil is a typical unconventional resource, which has the characteristics of good fluid properties and poor reservoir properties. The permeability and porosity of tight oil reservoir are general less than 1×103 μm2 and 10%, respectively [3]. Currently, the unconventional reservoirs are usually developed by horizontal wells, especially the segmented multi-cluster fractured horizontal wells, which have been widely used worldwide [4]. The simulated reservoir volume (SRV) can be formed around the horizontal well after segmented multi-cluster fracturing [5,6]. The technology of SRV is to achieve the important goal of increasing the contact area between matrix and fractures or fracture network as far as possible during the development of unconventional reservoirs [7,8]. The study and application of developing tight oil and shale oil reservoirs mostly focus on the natural depletion [9]. But some studies have shown that CO2 flooding is an effective approach of enhanced oil recovery (EOR) in tight reservoirs [6,10,11]. Based on the real case of Changqing tight oil reservoir in Ordos Basin, both PVT experiments and minimum miscible pressure (MMP) experiments were conducted for the crude oil sample of typical block. The compositional numerical model of typical five-spot well group is built, which is used to study the water flooding, CO2 flooding and Water-Alternate-Gas (WAG) of CO2 flooding. Different completion measures of horizontal well are analyzed. Furthermore, parameters such as the formation pressure, production rate, shut-in gas-oil ratio and total gas injection volume are analyzed. “Gas Flooding,” also known as miscible flooding, is one of the leading enhanced oil recovery (EOR) technologies employed for recovering oil that was formerly referred to as either stranded or trapped. Gas flooding is an “enhanced oil recovery” application for injecting miscible (and immiscible) gases into an oil reservoir for increasing oil production.

Gas flooding typically includes CO2, natural gas or nitrogen as the gas that is injected. Gas flooding tasked place as either a miscible flood or an immiscible flood. Miscible means that the gas that is injected “mixes” with the oil, thereby reducing viscosity and interfacial tension of the oil and rock. Miscible gas flooding also increases oil “swelling” and localized pressure or drive within the reservoir. “Immiscible” flooding means that the gas that is injected into the reservoir does not mix or go into solution. Therefore the purpose of the immiscible flood is to provide the energy or drive by increased pressure. Immiscible flooding does not produce as much oil as miscible gs flooding, however there are certain applications and reservoirs wherein immiscible flooding is well-suited.

Reservoir Characteristics and Numerical Simulation Model

PVT and MMP experiments

A crude oil sample is obtained in the tight oil reservoir. The PVT experiments are also conducted, and PR3 equation of state (EOS) is applied in PVT regression via PVTi module of Eclipse 2010 to match the experimental data of single flash vaporization test, differential liberation (DL) experiment and constant composition expansion (CCE) experiment. 9 pseudo-components of crude oil are grouped and their mole fractions are shown in Table 1. The parameters are well matched to meet the accuracy required for simulation (Table 2). It illustrates that the critical parameters of fluid can reflect the characters of real reservoir fluid.

Slim tube experiment of the crude oil sample is conducted. The result shows that the CO2 minimum miscible pressure (MMP) of the crude oil and CO2 is 19.8 MPa (Figure 1).

Numerical simulation model

Based on the tight oil reservoir, three numerical models of fivespot well groups with SRV are built. Model 1, a perforated horizontal well without fracturing located in the middle of the well group and the length of the horizontal section is 540 m (Figure 2, model 1). Model 2, there are a hydraulic fracturing horizontal well with three bi-wing transverse fractures in the toe, middle and heel of the well, respectively (Figure 2, model 2), and the location and length of model 2 are the same as model 1. Model 3, a 540 m length horizontal well located in the middle of the well group. The horizontal well has been segmented multi-cluster fractured and formed four 100 m × 180 m × 10 m SRVs around the wellbore (Figure 2, model 3). Four vertical wells are located around them for each model. In this paper, the geometry of fracture extension is assumed to be wire-mesh networks, which forms a rule rectangular fracture network after fracturing. In order to guarantee the convergence in simulation and achieve the rectangular fracture propagation, the grids with width of 0.1m after local grid refinement (LGR) are set to fractures. Vertical wells are also volume fractured to increase their injectivity, which is simulated simply by changing the reservoir permeability around them. Detailed parameters of these numerical models are shown in Table 3.

Development Scenarios Optimization

Different completion measures, including perforated completion, conventional fracturing completion and segmented multi-cluster fracturing completion, are analyzed firstly. Then the optimized well group with superior completion are used to optimize different development modes, such as water flooding, succession CO2 flooding and WAG of CO2 flooding. Finally, the parameters, including the formation pressure, production rate, shut-in gas-oil ratio and total gas injection volume, are optimized. Evaluation index is mainly the recovery factor of ten years and the CO2-oil draining efficiency. The CO2-oil draining efficiency is calculated by the following equation.

(1)

Where, is ECO2 CO2-oil draining efficiency, which is the reciprocal of CO2 utilization factor, is total oil production amount (t), T CO2 is CO2 total injection amount (t).

Well completion measures

In order to study the different horizontal well completions impact on development effect, three scenarios are designed. The first scenario is that the horizontal well is completed by perforating. The second scenario is that the horizontal well is completed by conventional fracturing. The third scenario is that the horizontal well is completed by segmented multi-cluster fracturing. The vertical wells are water injection wells, the horizontal wells are production wells, and the production control conditions of the three scenarios are the same.

As it can be seen from the Figure 3, the scenario of the horizontal well with SRV has the highest oil recovery, the following is the scenario of the horizontal well with bi-wing fracture, and the scenario of the horizontal well with perforation completion has the lowest recovery. Therefore, the horizontal well with SRV is selected as one of the best well completion measures to develop tight oil reservoir. In the following scenarios, the wells are all volume fractured as wells in model 3.

Development mode

The depletion development mode is usually used to develop tight oil reservoir, which is mainly due to the difficulty in injecting an oildisplacing agent to such tight reservoirs. The vertical and horizontal wells after volume fracturing can obtain SRV around them. The permeability and flow capacity of the reservoir have been greatly improved, which makes it possible to inject an oil-displacing agent to develop tight oil reservoir. Therefore, the water flooding, succession CO2 flooding and WAG of CO2 flooding are designed to study their effect on the development tight oil reservoirs. For succession CO2 flooding and WAG of CO2 flooding, the same amounts of CO2 are controlled to inject in these two scenarios.

In Figure 4, the scenario of WAG of CO2 flooding has the highest oil recovery, while the scenario of water flooding has the lowest recovery. The oil recovery of WAG of CO2 flooding is higher than that of the succession CO2 flooding. One reason is that the same CO2 amounts are injected in these two scenarios, and both of CO2 and water can contribute to oil recover in WAG of CO2 flooding, but there only CO2 contribute to oil recover in succession CO2 flooding. The other reason is that after gas breakthrough, the WAG of CO2 flooding can control the produced gas-oil ratio better than that of succession CO2 flooding. Considering obtaining the same recovery, it requires fewer CO2 amount of the WAG of CO2 flooding than that of succession CO2 flooding, which reduces the cost of gas flooding. Furthermore, the WAG of CO2 flooding is better to maintain formation pressure and to reduce the produced gas-oil ratio, so it is selected as the best development mode (Figure 5).

Production rate

As the production of horizontal well with SRV is controlled by flowing bottom-hole pressure (FBHP), different production rate can be obtained by adjusting the FBHP. In order to study the impact, 4 scenarios with different FBHP are designed, FBHP of which are 8 MPa, 9.3 MPa, 10 MPa and 12 MPa, respectively.

Figure 6 shows that the higher production rate (the lower the bottomhole pressure) is, the higher the corresponding oil recovery could obtain. But when the FBHP of production well is less than the saturation pressure, the increased of oil recovery is not that obvious, and the CO2- oil draining efficiency is greatly reduced. Also, the gas breakthrough time would become earlier. Furthermore, it is difficult to maintain the formation pressure around the MMP in the later stage. Therefore the FBHP should not be lower than the saturation, in the other words, the production rate should not be too large. Through the comprehensive analysis of the above, it is preferable to control bottomhole pressure at 9.3 MPa, which is little above saturation pressure (9.25 MPa). When the displacement front reaches the SRV of the horizontal well, the CO2 starts to breakthrough, then it should be appropriate to increase the water slug to decrease produced gas-oil ratio and maintain formation pressure.

CO2 injection amount

Based on these following 10 WAG of CO2 flooding scenarios, the relationship between oil recovery of ten years and CO2 injection amount is obtained. Figure 6 shows that CO2 injection volume is proportional to the oil recovery, the larger the CO2 injection amount is, the higher the oil recovery could obtain.

Ensuring adequate CO2 gas injection amount is the key to improve oil recovery. Figure 6 shows that oil recovery increases with the increasing of CO2 injection amount. But when the CO2 injection amount is larger than 4×104 t, the increase of oil recovery is slow. Therefore, the reasonable total injection amount of 4-4.5×104 t CO2 is recommended for this five-spot pattern well group.

Conclusion

The permeability and flow capacity of the reservoir have been greatly improved after larger scale SRV measures, which makes it possible to inject an oil-displacing agent to develop of tight oil reservoir. The WAG of CO2 flooding has better development effect than water flooding or succession CO2 flooding. The WAG of CO2 flooding can offset the short comings of low oil displacement efficiency of water flooding, and it can also improve the low sweep efficiency of succession CO2 flooding. In order to slow down the speed of the CO2 breakthrough and maintain the formation pressure, it should be appropriate to increase the water slug when the displacement front reaches the SRV of the horizontal wells. The MMP of CO2 and crude oil is a key factor for the WAG of CO2 flooding, it is better to maintain the formation pressure 1-2 MPa higher than MMP. CO2 injection volume is proportional to the oil recovery, but there is an optimal value.

Acknowledgements

This work was supported by the National Basic Research Program of China “Carbon Dioxide Emission Reduction, Storage and Resource Utilization (973 Program, Grant 2011CB707302)”, the Chinese National Major Science and Technology (Projects 2011ZX05016-006 and 2011ZX05009-004-001), and the Specialized Research Fund for the Doctoral Program of Higher Education of China (No 20120007120007).